1. Field of the Invention
This invention relates to a method of manufacturing dual phase strip steel and steel strip manufactured by the method. In particular the invention relates to a method of manufacturing a dual phase steel in the form of a strip of thickness in the range 0.1 to 0.5 mm from an unalloyed low C, low Mn steel composition having by weight
0.02-0.15% C PA1 0.15-0.50% Mn PA1 hot rolling PA1 cold rolling PA1 continuous annealing PA1 0.02-0.15% C PA1 0.15-0.50% Mn PA1 not more than 0.02% P PA1 not more than 0.03% Si PA1 not more than 0.065% Al.sub.as PA1 not more than 0.02% S PA1 not more than 50 ppm N PA1 balance Fe and unavoidable impurities.
comprising the steps of
the continuous annealing comprising
(a) heating the strip into the A.sub.1 -A.sub.3 region of the iron-carbon diagram and soaking it in said region and thereafter
(b) cooling the strip sufficiently rapidly that austenite is at least partly converted to martensite and/or bainite. Steel strip of this thickness is known as packing steel, because it may be used for various packaging functions, e.g. as tinplate.
2. Description of the Prior Art
A method as outlined above is disclosed in NL-A-8512364 which is discussed below.
Dual phase steels are now well known, and their production by continuous annealing is also well known. Dual phase steel is available either hot rolled, in a thickness of approximately 1.5-100 mm, or cold rolled, in a thickness of approximately 0.8-3 mm. See for example No. WO-79/00644 and EP-A No. 53913 which relates to steels for automotive applications (i.e. 0.8 mm thick in practice) and disclose steels which contain the alloying elements P and Si.
However, the production of thin strip of dual phase steel, i.e. with a thickness of 0.1-0.5 mm, presents a problem, because the known methods from producing the steel in greater thicknesses cannot be directly applied. One difficulty is to maintain the flatness of the strip.
Typically in the production of a strip of dual phase steel, the steel is quenched in cold water after heating in the continuous annealing line. During this cooling the cooling rate may be 1000.degree. C./sec. for a strip thickness of 1 mm. The cooling rate is inversely proportional to the thickness of the strip. Thus cooling a 1 mm thick strip at 1000.degree. C./sec. represents a P value of 1000 mm .degree.C./sec. where P is the product of cooling rate and strip thickness. If quenching in cold water is used as a cooling process for steel 0.1 to 0.5 mm thick, the strip will not remain flat because of thermal stresses, with the result that no strip of acceptable shape can be obtained.
NL-A No. 6512364 describes the production of thin strip of dual phase steel using cold water quenching, but it appears that the product obtained was not flat since in the examples given the product is subjected to a further rolling to make it flat. This is undesirable not only because of the cost of an extra step but also because the rolling introduces stresses which will cause further difficulties when the strip is cut.
Other cooling processes are known in the art and are likely to reduce or avoid the problems relating to strip shape where thin material is treated, e.g. gas (air) jet cooling with a P value of about 10 mm .degree./sec., or quenching in hot water with a P value of about 25 mm .degree.C./sec. However another difficulty then presents itself, which is to ensure the desired production of only or mainly martensite and/or bainite when using unalloyed low C, low Mn steel. With known treatments, this is achieved only if the strip is heated high in the A.sub.1 -A.sub.3 range, e.g. at about 850.degree. C., in the continuous annealing line. At such high temperatures strip fracture frequently occurs. Under the influence of the tensile force required for passing the strip through the continuous annealing line, the strip then collapses because of the low value of the yield point at that high temperature and the small supporting cross section of the thin material.
Strip fracture is very disadvantageous in continuous annealing. Not only is it very time consuming to feed the strip through the continuous annealing line again, with the resultant production loss, but strip material is lost when the continuous annealing line is restarted, until the desired process conditions are restored.